Field of the Invention
This invention relates to a process for the formation of an undercoat useful in forming a metal back layer, which makes use of a metal such as aluminum, on a fluorescent screen of a cathode ray tube (hereinafter abbreviated as "CRT").
Description of the Related Art
CRTs are used as displays for various applications. In keeping abreast of the diversification and ever-higher density of information in recent years, still higher performance, especially high-definition display, is required. As definition becomes higher, however, luminance drops so that in practice a limitation is obviously imposed on the degree of definition achievable.
To avoid reduction in luminance while providing higher definition, a technique has heretofore been employed in which, subsequent to the formation of a fluorescent layer, a metal back layer using a metal such as aluminum is formed to make use of its reflection.
According to the technique, in general, a film is formed on a fluorescent layer by using an emulsion of a resin in water, followed by the formation of a metal back layer on the film. As an alternative, a water film is formed on a fluorescent layer, a resin dissolved in a solvent is applied as a resin film on the water film, and a metal back layer is then formed on the resin film.
Of the conventional two processes described above, the process which uses an emulsion for the formation of a metal back layer is accompanied by a limitation on the degree of smoothness of the metal back layer to be formed because a great deal of gas evaporates during baking and this causes the metal back layer to bulge out from the fluorescent layer due to the evaporating gas (i.e., the so-called "blister"). On the other hand, the process which uses a resin dissolved in a solvent for the formation of a metal back layer is accompanied by such drawbacks as a water film cannot be formed easily or, even when formed, the water film lacks smoothness because the fluorescent layer has water repellency. In addition, it is difficult to form a uniform layer of solvent-base resin on a water film. This process involves practical problems such as the formation of irregularities, pinholes or cracks on the film surface.
In particular, a formation of a water film considerably affects the smoothness of a solvent-base resin layer to be formed on the water film. Many proposals have heretofore been made in this regard, for example, the formation of a thin film of an organic, high molecular substance subsequent to wetting a fluorescent layer with an aqueous solution of polyvinyl alcohol or gum arabic (Japanese Patent Publication No. 25659/1968), and the use of an aqueous solution of water-glass, said solution having been adjusted to a particular pH (Japanese Patent Laid-Open No. 232528/1986); and, as materials for attaining the above object, alcohols (Japanese Patent Laid-Open No. 4476/1974), cellulose derivatives, alginic acid derivatives or polyethylene oxide (Japanese Patent Laid-Open No. 192243/1983), dihydric or trihydric alcohols (Japanese Patent Laid-Open No. 218735/1985), and saturated aqueous solutions of water-soluble solvents (Japanese Patent Laid-Open No. 195540/1986).
None of the above water-film-forming compositions are however sufficient when evaluated from industrial viewpoints such as workability or when judged with respect to the performance required in recent years, i.e., the need for high definition and high luminance. No composition has heretofore been available, which can facilitate the formation of a solvent-base resin layer with sufficient smoothness while retaining high wetting power to a fluorescent screen.
Upon formation of a solvent-base resin film on a water film, higher luminance requires higher smoothness of the resin film. Further, as the area of the solvent-base resin film increases, more gas is produced in total during baking so that the thickness of the film must be reduced. This however results in a very large area-to-thickness ratio, thereby making it difficult to form a film. No composition has therefore been obtained for the formation of a solvent-base resin film suited to both high luminance and high definition.
As a factor which makes the formation of such an undercoat more difficult, the formation of a thin film on a water film, namely, the reliance upon the wet-on-wet method is mentioned. The formation of a thin, solvent-base resin film on a water film involves certain technical difficulties: because of their lower viscosities, interfacial disturbance tends to occur upon coating; the water film must be maintained at a predetermined height relative to the fluorescent screen during coating; and the formation of the film ought to be conducted while taking into due consideration the drying rates of the two films and consequent viscosity variations.